The star visited the Caribbean nation last month (Jan10) with U.S. politician John Edwards to hand out food and supplies to the survivors of the 12 January (10) disaster.
He was so upset by the scenes of devastation, he set up the Jenkins-Penn Haitian Relief Organisation (JPHRO) in a bid to gather further aid and raise funds.
And Penn has warned the devastation in Haiti will worsen when the rainy season hits the country - in just two weeks time.
During an appearance on U.S. talk show Larry King on Monday night (15Feb10), he says, "This is the Apocalypse. This is like (the effects of nuclear disaster) Hiroshima. The devastation there is on a level like nothing anyone that I've spoken to has ever seen.
"We're within two weeks from the rainy season. People have in essence no cover. There are tent cities that are made from rugged tarps on top and sheets on the side. It's an incredible fire hazard. The camps are wildly overcrowded, a breeding ground for infectious diseases.
"So what has to happen in the next two weeks... is to try to relocate as many as possible. People are going to die in mass if we don't get those camps closed."
But Penn is full of praise for the strong spirit of the Haitians in the aftermath of the disaster - and is planning to fly out to the country again later this week (19Feb10).
He adds, "The people are probably the most resilient - certainly the most I've ever experienced. We saw amputations without flinching, without anaesthesia.
"These are our neighbours, and this is a country that needs us and ultimately we need them because the character of the Haitian people is seeded in a disconnect from the comfort addiction that so many of us in the country have - that's a powerful force of character and that we need to share. I'm going back. I'll be back in Haiti on Friday."